Textile fibers are processed through a series of steps for conversion from the bale to the finished yarn or thread (hereafter referred to as yarn). Fiber from the bale is opened and cleaned and formed into a feed mat which is introduced into a carding machine. Here the fiber is further cleaned and straightened by the carding process and a thin web of the fibers is produced. The web is compressed widthwise to form a silver and the next step involves drawing or drafting the silver so as to increase its length and decrease its diameter. This drafting step decreases the weight of sliver per unit length prior to the twisting or spinning steps which typically occur next in the production of yarn.
Certain fibers, particularly cotton fibers, are limited in the fiber length from one (1) to one and one-half (11/2) inches and are typically one and one-eighth (11/8) inches, in length. The ultimate strength of a yarn made up of many fibers depends on the frictional force between adjacent fibers acting to prevent tension applied to the yarn from pulling the fibers apart in the lengthwise direction. Thus, the strength of the yarn relies upon the character of the fiber surface (coefficient of friction), the force pressing adjacent fibers together (degree of twist) and the length of contact between adjacent fibers.
At the stage of drafting silver prior to beginning to twist the silver into yarn, the twist is negligible and so the inter-fiber force is minimal. Therefore the strength of the silver is dependent primarily on the coefficient of friction and the length of contact between adjacent fibers.
Drafting is the operation of extending the silver by driving a secondary pair of opposed silver conveying rollers which are downstream of a draft zone somewhat faster than a primary pair of opposed silver conveying rollers which are upstream of the draft zone. In the prior art drafting operations and equipment, control of relative silver weight per unit length from section to section of a silver was difficult, and breakages of the silver, requiring a rethreading of the equipment, occurred frequently.
The design of a drafting mechanism (known as a draw box) involves two basic decisions namely what should be the diameter of the rollers and what should be the spacing between the rollers. Since it is generally considered desirable to position rollers close together to maintain control of the silver, the question of roller diameter becomes the major decision. The optimum spacing of the rollers is considered to be in the range of 11/2 times the fiber length extending between the primary and the secondary roller/silver nip points. It is further accepted textile theory that a roller set having relatively large diameter rollers is preferred because large diameter rollers provide a shallower entry angle and a longer contact with the silver being drawn. However, making rollers larger in diameter, according to conventional design, requires greater spacing. Therefore, the two preferences of closer spacing and larger diameters are somewhat contradictory. The novel principles embodied in the present invention enable the machine designer to significantly overcome this conflict.
U.S. Pat. No. 4,823,597 for a SLIVER MEASURING APPARATUS and U.S. Pat. No. 4,947,947 for a SLIVER MEASURING APPARATUS WITH OVERLOAD RELIEF, issued to the present inventor describe apparatus incorporating a funnel guide through which the silver passes and which mounts on a plate having a strain gage. The apparatus determines the relative bulk of the silver by its resistance to passage through the funnel. The resultant strain gage output signal is used to indicate the weight per unit length of silver based on which the draft ratio can be regulated. The teachings of the '597 and '947 patents are to be deemed incorporated herein by reference.
U.S. Pat. No. 2,975,501 to Whitmore discloses a roller arrangement in which a second pair of rollers is oriented substantially perpendicular to a first pair of rollers.
A further aspect of the present invention involves an improved non-contacting silver position detection unit. One prior device used to detect the silver position without contacting the silver requires use of a pair of parallel-mounted signal emitter-photosensitive cells connected to a logic device to determine silver presence in a target range. A second known device for the same purpose uses a plurality of such photocells and detectors mounted in a planar array to define a larger target zone. Neither of the mentioned prior silver detection devices have the advantages of the present invention.
It is therefore an object of this invention to provide an improved draft apparatus in which the distance between the nips of sequential roller sets is minimized.
It is a further object of this invention to provide an improved draft apparatus with improved means for controlling the weight per unit length of the silver being processed.
It is an additional object of this invention to provide an improved draft apparatus able to produce a substantially uniform drawn silver product.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.